Is the inter ocular determined by subject distance? Or is it more of a black art combination of subject size, distance and focal length?
There is no black art, Kevin. A couple of years ago I spent a lot of time on stereoscopy, even designed a camera prototype. Here are some notes based on your specific case:
1. The Interaxial distance is something that can change based on how you want the elements of the scene to be placed in the 'depth' (think: box) you create. You might come across 'rules' such as keeping the interaxial distance greater than the interocular distance for far away objects. In your case, depending on your height above ground, this might apply. If this is all you're shooting, I suggest a custom-made (two camera) side-by-side setup. Why custom-made? Because I'm not sure consumer cameras can stretch the interaxial distance to the levels you might need.
2. If you are too high above ground, you also run the risk of eliminating the stereoscopic effect entirely, as you can see with the human eye. After a certain distance, our stereoscopic ability vanishes and objects far away (not really that far in terms of feet, mind you) compress to a 2D plane. Same with camera lenses. And if you try to force the issue, then objects might start to look like miniatures.
3. Another 'rule' professionals follow is to NOT set convergence while shooting, for many technical reasons. I suggest you start reading here: http://www.dashwood3d.com/blog/beginners-guide-to-shooting-stereoscopic-3d/
and if you are really serious you HAVE to study these two books:
- Foundations of the stereoscopic cinema by Lenny Lipton (probably the Ansel Adams of the stereoscopic world)
- 3D Movie Making by Bernard Mendiburu
4. As to why stereoscopy hasn't caught on, regardless of the hype, marketing and Hollywood films, the answer is pretty simple: You can only make one kind of 'depth box' for each type of 'screen' (You can make two kinds if you use three cameras, but that is another nightmare altogether). What this means, is that if you apply the interaxial distance and convergence calculations (plus a host of other post-production calculations) for a particular screen type, like a cinema screen, for example, then the 3D won't work for a smaller screen type. What's worse, the distance of the viewer from the screen (and the angle) is also very important. There are too many variables that are beyond the filmmaker's control. Multiplexes usually conform to the SMPTE or THX standard, and the viewing angle and distances are controlled. This allows filmmakers to estimate the 3D experience (just like they do with audio) within acceptable tolerance levels. But at home, all this goes out the window.
Add to this the discomfort of wearing glasses, and having enough glasses for everyone watching at home! As far as I know, the only successful market penetration has been in the gaming world, where the experiences are controlled, and the 3D variables can be 'generated and manipulated on the fly' based on the systems used. There are really cool autostereoscopic displays out there, but then again, the quantity of quality content isn't there yet.
What Hollywood studios usually do is make the film for cinema and then 'redo' the 3D in post production (a compromise at best, since studios can't control the viewer's screen size, and the interaxial and/or convergence values have been set in stone already). Even if one is making 3D for the Internet, DVD or Blu-ray exclusively, the screen size can range from 10" to 60". Lower frame rates don't help either, which is why PJ roots for 48fps (a compromise for wide release), Cameron fights for 60fps (He hopes enough theaters will be ready by then), and Douglas Turnbull backs 120fps. So far, nobody has an answer to this problem.
5. Add to all this the different technologies out there, from anaglyph to Dolby to Real3D to nVidia ad infinitum. Every technology 'renders' 3D differently, and what works on one might not work on the other!
Please understand that my intention is not to discourage you, but to help you realize it's a field where a lot of effort, dedication, study and commitment is involved. You can't fake it, and you can't copy-paste.
If you do manage to wade through the muck and find solutions to your own problems, I think, ultimately, it is a worthwhile endeavor. And it's a lot of fun once you get the hang of it. There are many resources on the internet, and lots of support so you'll never get stuck. I hope this helps to get you started.